1. Field of the Invention
The invention relates to transparent and in particular glass substrates, which are provided with an antireflection coating, as well as to their production method. It also relates to the use thereof, particularly as glazings.
2. Discussion of the Background
An antireflection coating is usually formed by a stack of thin interface layers, generally an alternation of low and high refractive index layers. When deposited on a transparent substrate, such a coating has the function of reducing its light reflection, i.e. of increasing its light transmission. Thus, a substrate coated in this way is subject to an increase in its transmitted light to reflected light ratio, which improves the visibility of objects positioned behind it.
An antireflection coating can then be used in numerous applications, e.g. for protecting a panel illuminated by a light placed behind the observer, or for forming or constituting part of a shop display window, so as to make it easier to see what is in the window, even when the internal illumination is weak compared with the external illumination.
The performance characteristics of an antireflection coating can be measured or evaluated on the basis of different criteria. Clearly the fist criteria are of an optical nature. It can be considered that a xe2x80x9cgoodxe2x80x9d antireflection coating must be able to lower the light reflection of a standard clear glass substrate to a given value, e.g. 2%, or even 1% and less. It can also be important that the coating ensures that the substrate retains a satisfactory, e.g. neutral calorimetry, very close to that of the bare substrate. Other secondary criteria can be taken into account as a function of the envisaged application, particularly the chemical and/or mechanical durability of the coating, the cost of the materials used or the methods to be used for producing the same.
Patent application WO-92/04185 discloses an antireflection coating deposited on a transparent substrate and constituted by an alternation of layers having a high niobium oxide index and a low silicon oxide index. Its optical performance characteristics are interesting. It is advantageous to use niobium oxide from the industrial standpoint, because it is a material which can be deposited faster than other high index oxides of the titanium oxide type using known vacuum methods, such as reactive cathodic sputtering. However, it is found that such a stack is sensitive to any heat treatment and at high temperature its optical properties are unfavorably modified, particularly with respect to its colorimetry in reflection. This is disadvantageous if it is wished to give the particular substrate already provided with its coating mechanical or esthetic properties which can only be obtained by heat treatments at temperatures which may approach the softening temperature or point of the glass. Such treatment can, e.g., consist of bending or giving the substrate a certain curvature, an annealing for hardening it, or a tempering to prevent injury in the case of shattering.
One object of the invention is to obviate this disadvantage by developing a new type of antireflection, multilayer coating, which has good optical performance characteristics and which retains the latter, no matter whether or not the substrate then undergoes a heat treatment.
The invention relates to a glass substrate having on at least one of its face an antireflection coating incorporating a stack of thin layers of dielectric materials with alternatively high and low reflective indices. The invention prevents modification to the optical properties of the coating in the case where the substrate is subject to a heat treatment of the tempering, bending or annealing type by ensuring that the layer or layers of the stack which may be subject to deterioration in contact with alkali metal ions, for example of the sodium ion type, emitted by diffusion of the substrate are separated from said substrate by at least one layer forming part of the antireflection coating and which forms a xe2x80x9cshieldxe2x80x9d to the diffusion of the alkali ions.
Thus, it has surprisingly been found that the unfavorable modification of the optical appearance of antireflection coatings under the effect of heat was due to the diffusion of alkali ions from the glass, the ions being inserted in at least some of the layers of the coating thereby structurally modifying these layers leading to a deterioration thereof. The solution according to the invention involves not removing from the antireflection, coating of any material sensitive to the alkali ions, but instead isolating the same from the surface of the glass by means of a shielding layer blocking the alkali diffusion process. This layer is also chosen so as to fulfill, in parallel, an adequate optical function within the antireflection coating. Thus, it is not an additional layer which makes the structure of a conventional antireflection coating more complicated, which is very advantageous from the industrial standpoint.
Thus, these shielding layers make it possible to produce antireflection coatings able to withstand heat treatments without any significant optical modification, while incorporating materials sensitive to alkali, but offering many other advantages.